1. Field of the Invention
The present invention relates to a servo device of an optical disk unit for optically recording and reproducing information and an improvement of a servo system using all closed-loop control systems. More particularly, the present invention relates to a closed-loop control system and a servo device of an optical disk unit for suitably correcting a loop gain by using a simple structure.
2. Description of the Related Art
A general optical disk unit for optically recording and reproducing information has a known tracking servo controller for exactly making a light beam follow up an information track on an optical disk. For example, such a tracking servo controller is shown in Japanese Patent Application Laying Open (KOKAI) No. 2-192036.
An overshooting amount of a servo gain is increased when the servo gain is greater and smaller than an optimum value. Accordingly, there is a case in which no servo gain can be set to the optimum value from the above overshooting amount.
Further, it takes time to find and recorrect servo retracting timing when the servo gain is repeatedly set until the optimum value of the servo gain can be obtained.
In this general tracking servo controller, signal components mixed into a measured tracking error signal are simultaneously measured when the tracking error signal is measured. For example, these signal components are constructed by a track eccentric component, a disturbance vibrating component, a signal component disturbed by an address section of the optical, disk, other noises, etc. Therefore, it is impossible to suitably correct the servo gain by the simultaneous measurement of such signal components.
The inventors of this patent application proposed a servo circuit of a disk unit as one method for solving such a problem of the tracking servo controller.
In this servo circuit, a signal having a predetermined frequency is added to a servo loop and a servo loop gain is corrected by using amplitude ratios before and after this addition.
In the above servo circuit, the loop gain can be suitably corrected, but it is necessary to dispose a means for generating a signal having a specified frequency.
Therefore, there is a problem that the construction of the servo circuit is slightly complicated.